Method and apparatus for estimating channel in wireless communication system

ABSTRACT

The present invention relates to a wireless communication system. A method for estimating a channel by a user equipment (UE) in a wireless communication system includes receiving information about at least one channel station information (CSI) configuration for reporting CSI of one of a plurality of serving cells, determining that the same precoding matrix is applied to a plurality of resource blocks when all of the at least one CSI configuration is configured to report a precoding matrix indicator (PMI) and a rank indicator (RI), and determining that the precoding matrix is applied to one resource block when CSI configuration configured not to report the PMI and the RI is present among the at least one CSI configuration, and estimating a channel based on a result of the determination.

This application is a 35 USC §371 National Stage entry of InternationalApplication No. PCT/KR2013/007721 filed on Aug. 28, 2013, which claimspriority to U.S. Provisional Application Nos. 61/695,289 filed on Aug.30, 2012, 61/751,238 filed on Jan. 10, 2013 and 61/752,407 filed on Jan.14, 2013, each of which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for estimating a channel,for determining that the same precoding matrix is applied to a pluralityof resource blocks when at least one channel state information (CSI)configuration is configured to report a precoding matrix indicator (PMI)and a rank indicator (RI) in a wireless communication system supportingcoordinated multi-point (CoMP).

BACKGROUND ART

Multiple input multiple output (MIMO) increases the efficiency of datatransmission and reception using multiple transmit antennas and multiplereceive antennas instead of a single transmission antenna and a singlereception antenna. A receiver receives data through multiple paths whenmultiple antennas are used, whereas the receiver receives data through asingle antenna path when a single antenna is used. Accordingly, MIMO canincrease a data transmission rate and throughput and improve coverage.

A single cell MIMO scheme can be classified into a single user-MIMO(SU-MIMO) scheme for receiving a downlink signal by a single UE in onecell and a multi user-MIMO (MU-MIMO) scheme for receiving a downlinksignal by two or more UEs.

Research on coordinated multi-point (CoMP) for improving throughput of aUE located at a cell boundary by applying improved MIMO to a multi-cellenvironment is actively performed. The CoMP system can decreaseinter-cell interference in a multi-cell environment and improve systemperformance.

Channel estimation refers to a procedure for compensating for signaldistortion due to fading to restore a reception signal. Here, the fadingrefers to sudden fluctuation in signal intensity due to multipath-timedelay in a wireless communication system environment. For channelestimation, a reference signal (RS) known to both a transmitter and areceiver is required. In addition, the RS can be referred to as a RS ora pilot signal according to applied standard.

A downlink RS is a pilot signal for coherent demodulation for a physicaldownlink shared channel (PDSCH), a physical control format indicatorchannel (PCFICH), a physical hybrid indicator channel (PHICH), aphysical downlink control channel (PDCCH), etc. A downlink RS includes acommon RS (CRS) shared by all user equipments (UEs) in a cell and adedicated RS (DRS) for a specific UE. For a system (e.g., a systemhaving extended antenna configuration LTE-A standard for supporting 8transmission antennas) compared with a conventional communication system(e.g., a system according to LTE release-8 or 9) for supporting 4transmission antennas, DRS based data demodulation has been consideredfor effectively managing RSs and supporting a developed transmissionscheme. That is, for supporting data transmission through extendedantennas, DRS for two or more layers can be defined. DRS is pre-coded bythe same pre-coder as a pre-coder for data and thus a receiver caneasily estimate channel information for data demodulation withoutseparate precoding information.

A downlink receiver can acquire pre-coded channel information forextended antenna configuration through DRS but requires a separate RSother than DRS in order to non-pre-coded channel information.Accordingly, a receiver of a system according to LTE-A standard candefine a RS for acquisition of channel state information (CSI), that is,CSI-RS.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method and apparatus for estimating a channel in a wirelesscommunication system.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Technical Solution

The object of the present invention can be achieved by providing amethod for estimating a channel by a user equipment (UE) in a wirelesscommunication system, the method including receiving information aboutat least one channel station information (CSI) configuration forreporting CSI of one of a plurality of serving cells, determining thatthe same precoding matrix is applied to a plurality of resource blockswhen all of the at least one CSI configuration is configured to report aprecoding matrix indicator (PMI) and a rank indicator (RI), anddetermining that the precoding matrix is applied to one resource blockwhen CSI configuration configured not to report the PMI and the RI ispresent among the at least one CSI configuration, and estimating achannel based on a result of the determination.

In another aspect of the present invention, provided herein is a methodfor receiving channel information by a base station (BS) in a wirelesscommunication system, the method including transmitting informationabout at least one channel station information (CSI) configuration forreporting CSI of one of a plurality of serving cells, applying the sameprecoding matrix applied to a plurality of resource blocks when all ofthe at least one CSI configuration is configured to report a precodingmatrix indicator (PMI) and a rank indicator (RI), and applying theprecoding matrix to one resource block when CSI configuration configurednot to report the PMI and the RI is present among the at least one CSIconfiguration, and receiving information about a channel to which theprecoding matrix is applied from a user equipment (UE).

In another aspect of the present invention, provided herein is a userequipment (UE) for estimating a channel in a wireless communicationsystem, the UE including a radio frequency (RF) unit, and a processor,wherein the processor is configured to receive information about atleast one channel station information (CSI) configuration for reportingCSI of one of a plurality of serving cells, to determine that the sameprecoding matrix is applied to a plurality of resource blocks when allof the at least one CSI configuration is configured to report aprecoding matrix indicator (PMI) and a rank indicator (RI), to determinethat the precoding matrix is applied to one resource block when CSIconfiguration configured not to report the PMI and the RI is presentamong the at least one CSI configuration, and to estimate a channelbased on a result of the determination.

In another aspect of the present invention, provided herein is a basestation (BS) for receiving channel information in a wirelesscommunication system, the BS including a radio frequency (RF) unit, anda processor, wherein the processor is configured to transmit informationabout at least one channel station information (CSI) configuration forreporting CSI of one of a plurality of serving cells, to apply the sameprecoding matrix applied to a plurality of resource blocks when all ofthe at least one CSI configuration is configured to report a precodingmatrix indicator (PMI) and a rank indicator (RI), to apply the precodingmatrix to one resource block when CSI configuration configured not toreport the PMI and the RI is present among the at least one CSIconfiguration, and to receive information about a channel to which theprecoding matrix is applied from a user equipment (UE).

The following features can be commonly applied to the embodiments of thepresent invention.

The number of the plural resource blocks may be determined based on asystem bandwidth when all of the at least one CSI configuration isconfigured to report the PMI and the RI.

The at least one CSI configuration may be independently configured withrespect to whether the PMI and the RI are reported.

Information about the at least one CSI configuration may be transmittedby radio resource control (RRC) signaling.

The method may further include receiving a demodulation reference signal(DM RS).

The estimating may include estimating a channel using the DM RS.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to embodiments of the present invention, channel estimationcan be more effectively reported in a wireless communication system.

According to the embodiments of the present invention, it may bedetermined that the same precoding matrix is applied to a plurality ofresource blocks when at least one channel state information (CSI)configuration is configured to report a precoding matrix indicator (PMI)and a rank indicator (RI) in a wireless communication system supportingcoordinated multi-point (CoMP).

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description taken in conjunction with theaccompanying drawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 illustrates the type-1 radio frame structure;

FIG. 2 illustrates the structure of a downlink resource grid for theduration of one downlink slot;

FIG. 3 illustrates the structure of a downlink subframe;

FIG. 4 illustrates the structure of an uplink subframe;

FIG. 5 illustrates the configuration of a MIMO communication systemhaving multiple antennas;

FIG. 6 illustrates a conventional CRS and DRS pattern.

FIG. 7 illustrates an exemplary DM RS pattern defined for the LTE-Asystem;

FIG. 8 illustrates exemplary CSI-RS patterns;

FIG. 9 illustrates an exemplary periodic CSI-RS transmission;

FIG. 10 illustrates an exemplary aperiodic CSI-RS transmission;

FIG. 11 illustrates an example of using two CSI-RS configurations;

FIG. 12 is a flowchart of a channel estimation method according to anembodiment of the present invention; and

FIG. 13 is a diagram illustrating a BS and a UE to which an embodimentof the present invention can be applicable.

BEST MODE

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. Also, some constituent components and/orcharacteristics may be combined to implement the embodiments of thepresent invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a terminal.In this case, the base station is used as a terminal node of a networkvia which the base station can directly communicate with the terminal.Specific operations to be conducted by the base station in the presentinvention may also be conducted by an upper node of the base station asnecessary.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the base station to communicate with theterminal in a network composed of several network nodes including thebase station will be conducted by the base station or other networknodes other than the base station. The term “base station (BS)” may bereplaced with a fixed station, Node-B, eNode-B (eNB), or an access pointas necessary. The term “relay” may be replaced with the terms relay node(RN) or relay station (RS). The term “terminal” may also be replacedwith a user equipment (UE), a mobile station (MS), a mobile subscriberstation (MSS) or a subscriber station (SS) as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an institute of electrical and electronics engineers (IEEE)802 system, a 3rd generation partnership project (3GPP) system, a 3GPPlong term evolution (LTE) system, an LTE-advanced (LTE-A) system, and a3GPP2 system. In particular, steps or parts, which are not described toclearly reveal the technical idea of the present invention, in theembodiments of the present invention may be supported by the abovedocuments. All terminology used herein may be supported by at least oneof the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, code divisionmultiple access (CDMA), frequency division multiple access (FDMA), timedivision multiple access (TDMA), orthogonal frequency division multipleaccess (OFDMA), single carrier frequency division multiple access(SC-FDMA), and the like. CDMA may be embodied through wireless (orradio) technology such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as global system for mobile communication (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMAmay be embodied through wireless (or radio) technology such as instituteof electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802-20, and evolved UTRA (E-UTRA). UTRA is a partof universal mobile telecommunications system (UMTS). 3rd generationpartnership project (3GPP) long term evolution (LTE) is a part of E-UMTS(Evolved UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA in downlinkand employs SC-FDMA in uplink. LTE-Advanced (LTE-A) is an evolvedversion of 3GPP LTE. WiMAX can be explained by IEEE 802.16e(wirelessMAN-OFDMA reference system) and advanced IEEE 802.16m(wirelessMAN-OFDMA advanced system). For clarity, the followingdescription focuses on IEEE 802.11 systems. However, technical featuresof the present invention are not limited thereto.

With reference to FIG. 1, the structure of a downlink radio frame willbe described below.

In a cellular orthogonal frequency division multiplexing (OFDM) wirelesspacket communication system, uplink and/or downlink data packets aretransmitted in subframes. One subframe is defined as a predeterminedtime period including a plurality of OFDM symbols. The 3GPP LTE standardsupports a type-1 radio frame structure applicable to frequency divisionduplex (FDD) and a type-2 radio frame structure applicable to timedivision duplex (TDD).

FIG. 1 illustrates the type-1 radio frame structure. A downlink radioframe is divided into 10 subframes. Each subframe is further dividedinto two slots in the time domain. A unit time during which one subframeis transmitted is defined as a Transmission Time Interval (TTI). Forexample, one subframe may be 1 ms in duration and one slot may be 0.5 msin duration. A slot includes a plurality of OFDM symbols in the timedomain and a plurality of resource blocks (RBs) in the frequency domain.Because the 3GPP LTE system adopts OFDMA for downlink, an OFDM symbolrepresents one symbol period. An OFDM symbol may be referred to as anSC-FDMA symbol or symbol period. An RB is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

The number of OFDM symbols in one slot may vary depending on a cyclicprefix (CP) configuration. There are two types of CPs: extended CP andnormal CP. In the case of the normal CP, one slot includes 7 OFDMsymbols. In the case of the extended CP, the length of one OFDM symbolis increased and thus the number of OFDM symbols in a slot is smallerthan in the case of the normal CP. Thus when the extended CP is used,for example, 6 OFDM symbols may be included in one slot. If channelstate gets poor, for example, during fast movement of a UE, the extendedCP may be used to further decrease inter-symbol interference (ISI).

In the case of the normal CP, one subframe includes 14 OFDM symbolsbecause one slot includes 7 OFDM symbols. The first two or three OFDMsymbols of each subframe may be allocated to a physical downlink controlchannel (PDCCH) and the other OFDM symbols may be allocated to aphysical downlink shared channel (PDSCH).

The above-described radio frame structures are purely exemplary and thusit is to be noted that the number of subframes in a radio frame, thenumber of slots in a subframe, or the number of symbols in a slot mayvary.

FIG. 2 illustrates the structure of a downlink resource grid for theduration of one downlink slot. FIG. 2 corresponds to a case in which anOFDM includes normal CP. Referring to FIG. 2, a downlink slot includes aplurality of OFDM symbols in the time domain and includes a plurality ofRBs in the frequency domain. Here, one downlink slot includes 7 OFDMsymbols in the time domain and an RB includes 12 subcarriers in thefrequency domain, which does not limit the scope and spirit of thepresent invention. An element on a resource grid is referred to as aresource element (RE). For example, RE a(k,l) refers to RE location in ak_(th) subcarrier and a first OFDM symbol. In the case of the normal CP,one RB includes 12×7 REs (in the case of the extended CP, one RBincludes 12×6 REs). An interval between subcarriers is 15 kHz and thusone RB includes about 180 kHz in the frequency domain. N^(DL) is numberof RBs in a downlink slot. N^(DL) depends on a downlink transmissionbandwidth configured by BS scheduling.

FIG. 3 illustrates the structure of a downlink subframe. Up to threeOFDM symbols at the start of the first slot in a downlink subframe areused for a control region to which control channels are allocated andthe other OFDM symbols of the downlink subframe are used for a dataregion to which a PDSCH is allocated. A basic unit of transmission isone subframe. That is, a PDCCH and a PDSCH are allocated across twoslots. Downlink control channels used in the 3GPP LTE system include,for example, a physical control format indicator channel (PCFICH), aphysical downlink control channel (PDCCH), and a physical hybridautomatic repeat request (HARQ) indicator channel (PHICH). The PCFICH islocated in the first OFDM symbol of a subframe, carrying informationabout the number of OFDM symbols used for transmission of controlchannels in the subframe. The PHICH delivers an HARQACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal in response toan uplink transmission. Control information carried on the PDCCH iscalled downlink control information (DCI). The DCI transports uplink ordownlink scheduling information, or uplink transmission power controlcommands for UE groups. The PDCCH delivers information about resourceallocation and a transport format for a downlink shared channel(DL-SCH), resource allocation information about an uplink shared channel(UL-SCH), paging information of a paging channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a random access responsetransmitted on the PDSCH, a set of transmission power control commandsfor individual UEs of a UE group, transmission power controlinformation, voice over Internet protocol (VoIP) activation information,etc. A plurality of PDCCHs may be transmitted in the control region. AUE may monitor a plurality of PDCCHs. A PDCCH is formed by aggregatingone or more consecutive Control Channel Elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE corresponds to a plurality of REgroups. The format of a PDCCH and the number of available bits for thePDCCH are determined according to the correlation between the number ofCCEs and a coding rate provided by the CCEs. An eNB determines the PDCCHformat according to DCI transmitted to a UE and adds a cyclic redundancycheck (CRC) to control information. The CRC is masked by an identifier(ID) known as a radio network temporary identifier (RNTI) according tothe owner or usage of the PDCCH. When the PDCCH is directed to aspecific UE, its CRC may be masked by a cell-RNTI (C-RNTI) of the UE.When the PDCCH is for a paging message, the CRC of the PDCCH may bemasked by a paging indicator identifier (P-RNTI). When the PDCCH carriessystem information, particularly, a system information block (SIB), itsCRC may be masked by a system information ID and a system informationRNTI (SI-RNTI). To indicate that the PDCCH carries a random accessresponse in response to a random access preamble transmitted by a UE,its CRC may be masked by a random access-RNTI (RA-RNTI).

FIG. 4 illustrates the structure of an uplink subframe. An uplinksubframe may be divided into a control region and a data region in thefrequency domain. A Physical Uplink Control Channel (PUCCH) carryinguplink control information is allocated to the control region and aphysical uplink shared channel (PUSCH) carrying user data is allocatedto the data region. To maintain the property of a single carrier, a UEdoes not transmit a PUSCH and a PUCCH simultaneously. A PUCCH for a UEis allocated to an RB pair in a subframe. The RBs of the RB pair occupydifferent subcarriers in two slots. Thus it is said that the RB pairallocated to the PUCCH is frequency-hopped over a slot boundary.

Modeling of MIMO System

A multiple input multiple output (MIMO) system increasestransmission/reception efficiency of data using multiple transmission(Tx) antennas and multiple reception (Rx) antennas. MIMO technology doesnot depend upon a single antenna path in order to receive all messagesbut instead can combine a plurality of data fragments received through aplurality of antennas and receive all data.

MIMO technology includes a spatial diversity scheme, a spatialmultiplexing scheme, etc. The spatial diversity scheme can increasetransmission reliability or can widen a cell diameter with diversitygain and thus is appropriate for data transmission of a UE that moves ahigh speed. The spatial multiplexing scheme can simultaneously transmitdifferent data so as to increase data transmission rate without increasein a system bandwidth.

FIG. 5 illustrates the configuration of a MIMO communication systemhaving multiple antennas. As illustrated in FIG. 5(a), the simultaneoususe of a plurality of antennas at both the transmitter and the receiverincreases a theoretical channel transmission capacity, compared to useof a plurality of antennas at only one of the transmitter and thereceiver. Therefore, transmission rate may be increased and frequencyefficiency may be remarkably increased. As channel transmission rate isincreased, transmission rate may be increased, in theory, to the productof a maximum transmission rate R_(o) that may be achieved with a singleantenna and a transmission rate increase Ri.R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For instance, a MIMO communication system with four Tx antennas and fourRx antennas may achieve a four-fold increase in transmission ratetheoretically, relative to a single-antenna system. Since thetheoretical capacity increase of the MIMO system was verified in themiddle 1990s, many techniques have been actively proposed to increasedata rate in real implementation. Some of the techniques have alreadybeen reflected in various wireless communication standards for 3G mobilecommunications, future-generation wireless local area network (WLAN),etc.

Concerning the research trend of MIMO up to now, active studies areunderway in many respects of MIMO, inclusive of studies of informationtheory related to calculation of multi-antenna communication capacity indiverse channel environments and multiple access environments, studiesof measuring MIMO radio channels and MIMO modeling, studies oftime-space signal processing techniques to increase transmissionreliability and transmission rate, etc.

Communication in a MIMO system will be described in detail throughmathematical modeling. It is assumed that N_(T) Tx antennas and N_(R) Rxantennas are present in the system.

Regarding a transmission signal, up to N_(T) pieces of information canbe transmitted through the N_(T) Tx antennas, as expressed in Equation 2below.s=└s ₁ ,s ₂ , . . . ,s _(N) _(T) ┘^(T)  [Equation 2]

A different transmission power may be applied to each piece oftransmission information, s₁, s₂, . . . , s_(N) _(T) . Let thetransmission power levels of the transmission information be denoted byP₁, P₂, . . . , P_(N) _(T) , respectively. Then the transmissionpower-controlled transmission information vector is given asŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T)=[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P _(N)_(T) s _(N) _(T) ]^(T)  [Equation 3]

The transmission power-controlled transmission information vector ŝ maybe expressed as follows, using a diagonal matrix P of transmissionpower.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) may be generatedby multiplying the transmission power-controlled information vector ŝ bya weight matrix W. The weight matrix W functions to appropriatelydistribute the transmission information to the Tx antennas according totransmission channel states, etc. These N_(T) transmission signals x₁,x₂, . . . , x_(N) _(T) are represented as a vector x, which may bedetermined by Equation 5 below.

$\begin{matrix}{x = {\quad{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix}{\quad{= {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}}}}} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

Here, w_(ij) refers to a weight between an i_(th) Tx antenna and j_(th)information.

A reception signal x may be considered in different ways according totwo cases (e.g., spatial diversity and spatial multiplexing). In thecase of spatial multiplexing, different signals are multiplexed and themultiplexed signals are transmitted to a receiver, and thus, elements ofinformation vector (s) have different values. In the case of spatialdiversity, the same signal is repeatedly transmitted through a pluralityof channel paths and thus elements of information vectors (s) have thesame value. A hybrid scheme of spatial multiplexing and spatialdiversity can also be considered. That is, that same signal may betransmitted through three Tx antennas and the remaining signals may bespatial-multiplexed and transmitted to a receiver.

In the case of N_(R) Rx antennas, a reception signal of each antenna maybe expressed as the vector shown in Equation 6 below.y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

When a channel modeling is executed in the MIMO communication system,individual channels can be distinguished from each other according totransmission/reception (Tx/Rx) antenna indexes. A channel passing therange from a Tx antenna j to an Rx antenna i is denoted by h_(ij). Itshould be noted that the index order of the channel h_(ij) is locatedbefore a reception (Rx) antenna index and is located after atransmission (Tx) antenna index.

FIG. 5(b) illustrates channels from N_(T) Tx antennas to an Rx antennai. The channels may be collectively represented in the form of vectorand matrix. Referring to FIG. 5(b), the channels passing the range fromthe N_(T) Tx antennas to the Rx antenna i can be represented by theEquation 7 below.h _(i) ^(T)=[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

All channels passing the range from the N_(T) Tx antennas to N_(R) Rxantennas are denoted by the matrix shown in Equation 8 below.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

Additive white Gaussian noise (AWGN) is added to an actual channel whichhas passed the channel matrix. The AWGN (n₁, n₂, . . . , n_(NR)) addedto each of N_(R) reception (Rx) antennas can be represented by Equation9 below.n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

A reception signal calculated by the above-mentioned equations can berepresented by Equation 10 below.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{j}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \lbrack {{Equation}\mspace{14mu} 10} \rbrack\end{matrix}$

The number of rows and the number of columns of a channel matrix Hindicating a channel condition are determined by the number of Tx/Rxantennas. In the channel matrix H, the number of rows is equal to thenumber (N_(R)) of Rx antennas, and the number of columns is equal to thenumber (N_(T)) of Tx antennas. Namely, the channel matrix H is denotedby an N_(R)×N_(T) matrix.

The rank of a matrix is defined as the smaller between the number ofindependent rows and the number of independent columns in the channelmatrix. Accordingly, the rank of the channel matrix is not larger thanthe number of rows or columns of the channel matrix. The rank of achannel matrix H, rank(H) satisfies the following constraint.rank(H)≦min(N _(T) ,N _(R))  [Equation 11]

For MIMO transmission, ‘rank’ indicates the number of paths forindependent transmission of signals and ‘number of layers’ indicates thenumber of streams transmitted through each path. In general, atransmission end transmits layers, the number of which corresponds tothe number of ranks used for signal transmission, and thus, rank havethe same meaning as number of layers unless there is no differentdisclosure.

Reference Signals (RSs)

In a wireless communication system, a packet is transmitted on a radiochannel. In view of the nature of the radio channel, the packet may bedistorted during the transmission. To receive the signal successfully, areceiver should compensate for the distortion of the reception signalusing channel information. Generally, to enable the receiver to acquirethe channel information, a transmitter transmits a signal known to boththe transmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. This signal is called a pilot signal or an RS.

In the case of data transmission and reception through multipleantennas, knowledge of channel states between transmission (Tx) antennasand reception (Rx) antennas is required for successful signal reception.Accordingly, an RS should be transmitted through each Tx antenna.

RSs in a mobile communication system may be divided into two typesaccording to their purposes: RS for channel information acquisition andRS for data demodulation. Since its purpose lies in that a UE acquiresdownlink channel information, the former should be transmitted in abroad band and received and measured even by a UE that does not receivedownlink data in a specific subframe. This RS is also used in asituation like handover. The latter is an RS that an eNB transmits alongwith downlink data in specific resources. A UE can estimate a channel byreceiving the RS and accordingly can demodulate data. The RS should betransmitted in a data transmission area.

A legacy 3GPP LTE (e.g., 3GPP LTE release-8) system defines two types ofdownlink RSs for unicast services: a common RS (CRS) and a dedicated RS(DRS). The CRS is used for acquisition of information about a channelstate, measurement of handover, etc. and may be referred to as acell-specific RS. The DRS is used for data demodulation and may bereferred to as a UE-specific RS. In a legacy 3GPP LTE system, the DRS isused for data demodulation only and the CRS can be used for bothpurposes of channel information acquisition and data demodulation.

CRSs, which are cell-specific, are transmitted across a wideband inevery subframe. According to the number of Tx antennas at an eNB, theeNB may transmit CRSs for up to four antenna ports. For instance, an eNBwith two Tx antennas transmits CRSs for antenna port 0 and antennaport 1. If the eNB has four Tx antennas, it transmits CRSs forrespective four Tx antenna ports, antenna port 0 to antenna port 3.

FIG. 6 illustrates a CRS and DRS pattern for an RB (including 14 OFDMsymbols in time by 12 subcarriers in frequency in case of a normal CP)in a system where an eNB has four Tx antennas. In FIG. 6, REs labeledwith ‘R0’, ‘R1’, ‘R2’ and ‘R3’ represent the positions of CRSs forantenna port 0 to antenna port 4, respectively. REs labeled with ‘D’represent the positions of DRSs defined in the LTE system.

The LTE-A system, an evolution of the LTE system, can support up toeight Tx antennas. Therefore, it should also support RSs for up to eightTx antennas. Because downlink RSs are defined only for up to four Txantennas in the LTE system, RSs should be additionally defined for fiveto eight Tx antenna ports, when an eNB has five to eight downlink Txantennas in the LTE-A system. Both RSs for channel measurement and RSsfor data demodulation should be considered for up to eight Tx antennaports.

One of significant considerations for design of the LTE-A system isbackward compatibility. Backward compatibility is a feature thatguarantees a legacy LTE terminal to operate normally even in the LTE-Asystem. If RSs for up to eight Tx antenna ports are added to atime-frequency area in which CRSs defined by the LTE standard aretransmitted across a total frequency band in every subframe, RS overheadbecomes huge. Therefore, new RSs should be designed for up to eightantenna ports in such a manner that RS overhead is reduced.

Largely, new two types of RSs are introduced to the LTE-A system. Onetype is CSI-RS serving the purpose of channel measurement for selectionof a transmission rank, a modulation and coding scheme (MCS), aprecoding matrix index (PMI), etc. The other type is demodulation RS (DMRS) for demodulation of data transmitted through up to eight Txantennas.

Compared to the CRS used for both purposes of measurement such aschannel measurement and measurement for handover and data demodulationin the legacy LTE system, the CSI-RS is designed mainly for channelestimation, although it may also be used for measurement for handover.Since CSI-RSs are transmitted only for the purpose of acquisition ofchannel information, they may not be transmitted in every subframe,unlike CRSs in the legacy LTE system. Accordingly, CSI-RSs may beconfigured so as to be transmitted intermittently (e.g. periodically)along the time axis, for reduction of CSI-RS overhead.

When data is transmitted in a downlink subframe, DM RSs are alsotransmitted dedicatedly to a UE for which the data transmission isscheduled. Thus, DM RSs dedicated to a particular UE may be designedsuch that they are transmitted only in a resource area scheduled for theparticular UE, that is, only in a time-frequency area carrying data forthe particular UE.

FIG. 7 illustrates an exemplary DM RS pattern defined for the LTE-Asystem. In FIG. 7, the positions of REs carrying DM RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. DM RSs maybe transmitted for additionally defined four antenna ports, antenna port7 to antenna port 10 in the LTE-A system. DM RSs for different antennaports may be identified by their different frequency resources(subcarriers) and/or different time resources (OFDM symbols). This meansthat the DM RSs may be multiplexed in frequency division multiplexing(FDM) and/or time division multiplexing (TDM). If DM RSs for differentantenna ports are positioned in the same time-frequency resources, theymay be identified by their different orthogonal codes. That is, these DMRSs may be multiplexed in Code Division Multiplexing (CDM). In theillustrated case of FIG. 7, DM RSs for antenna port 7 and antenna port 8may be located on REs of DM RS CDM group 1 through multiplexing based onorthogonal codes. Similarly, DM RSs for antenna port 9 and antenna port10 may be located on REs of DM RS CDM group 2 through multiplexing basedon orthogonal codes.

FIG. 8 illustrates exemplary CSI-RS patterns defined for the LTE-Asystem. In FIG. 8, the positions of REs carrying CSI-RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. One of theCSI-RS patterns illustrated in FIGS. 8(a) to 8(e) is available for anydownlink subframe. CSI-RSs may be transmitted for eight antenna portssupported by the LTE-A system, antenna port 15 to antenna port 22.CSI-RSs for different antenna ports may be identified by their differentfrequency resources (subcarriers) and/or different time resources (OFDMsymbols). This means that the CSI-RSs may be multiplexed in FDM and/orTDM. CSI-RSs positioned in the same time-frequency resources fordifferent antenna ports may be identified by their different orthogonalcodes. That is, these DM RSs may be multiplexed in CDM. In theillustrated case of FIG. 8(a), CSI-RSs for antenna port 15 and antennaport 16 may be located on REs of CSI-RS CDM group 1 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 17 and antenna port18 may be located on REs of CSI-RS CDM group 2 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 19 and antenna port20 may be located on REs of CSI-RS CDM group 3 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 21 and antenna port22 may be located on REs of CSI-RS CDM group 4 through multiplexingbased on orthogonal codes. The same principle described with referenceto FIG. 8(a) is applicable to the CSI-RS patterns illustrated in FIGS.8(b) to 8(e).

The RS patterns illustrated in FIGS. 6, 7 and 8 are purely exemplary.Thus it should be clearly understood that various embodiments of thepresent invention are not limited to specific RS patterns. That is,various embodiments of the present invention can also be implemented inthe same manner when other RS patterns than those illustrated in FIGS.6, 7 and 8 are applied.

Cooperative Multi-Point (CoMP)

To satisfy enhanced system performance requirements for the 3GPP LTE-Asystem, CoMP transmission and reception technology known as co-MIMO,collaborative MIMO or network MIMO has been proposed. The CoMPtechnology can increase the performance of UEs located at a cell edgeand average sector throughput.

It is known that Inter-Cell Interference (ICI) generally degrades theperformance of a UE at a cell edge and average sector throughput in amulti-cellular environment with a frequency reuse factor of 1. To offeran appropriate throughput performance to a cell-edge UE in anenvironment constrained by interference, a simple ICI mitigationtechnique such as UE-specific power control-based Fractional FrequencyReuse (FFR) is used in the conventional LTE system. However, it may bepreferred to reduce the ICI or reuse the ICI as a desired signal for theUE, rather than to decrease the utilization of frequency resources percell. For this purpose, CoMP transmission techniques may be adopted.

Downlink CoMP schemes are classified largely into Joint Processing (JP),and Coordinated Scheduling/Beamforming (CS/CB).

According to the JP scheme, each point (eNB) of a CoMP unit may usedata. The CoMP unit refers to a set of eNBs used for a CoMP transmissionoperation. The JP scheme is further branched into joint transmission anddynamic cell selection.

Joint transmission is a technique of transmitting PDSCHs from aplurality of points (a part or the whole of a CoMP unit) at one time.That is, a plurality of transmission points may simultaneously transmitdata to a single UE. The joint transmission scheme can improve thequality of a received signal coherently or non-coherently and activelyeliminate interference to other UEs, as well.

Dynamic cell selection is a technique of transmitting a PDSCH from onepoint of a CoMP unit at one time. That is, one point of the CoMP unittransmits data to a single UE at a given time point, while the otherpoints of the CoMP unit do not transmit data to the UE at the timepoint. A point to transmit data to a UE may be dynamically selected.

Meanwhile, in the CS/CB scheme, a CoMP unit may perform cooperativebeamforming for data transmission to a single UE. While only a servingcell transmits data to the UE, user scheduling/beamforming may bedetermined through coordination among cells of the CoMP unit.

Uplink CoMP reception refers to uplink reception of a transmitted signalthrough coordination at a plurality of geographically separated points.Uplink CoMP schemes include Joint Reception (JR) and CS/CB.

In JR, a plurality of reception points receives a signal transmitted ona PUSCH. CS/CB is a technique in which while only one point receives aPUSCH, user scheduling/beamforming is determined through coordinationamong cells of a CoMP unit.

CSI-RS Configuration

For CoMP scheduling in a network, the UE needs to feedback DL CSIinformation of an adjacent cell that participates in CoMP as well DL CSIinformation of serving cell. To this end, the UE may feedback aplurality of CSI processes that reflect various data transmission celland various interference environments.

Thus, an LTE system uses an interference measurement resource (IMR) forinterference measurement during calculation of CoMP CSI. One UE may beconfigured by a plurality of IMRs which have independent configuration.That is, the IMRs may be configured by independent periods, offsets, andresource configuration, and a BS may signal IMR to a UE via higher-layersignaling (RRC, etc.).

In addition, an LTE system uses CSI-RS in order to measure a channeldesired for calculation of CoMP CSI. One UE may be configured by aplurality of CSI-RSs which have independent configurations. That is,each CSI-RS may be configured by independent periods, offsets, resourceconfiguration, power control (Pc), and number of antenna ports. CSI-RSrelated information may be signaled to a UE from a BS via higher-layersignaling (RRC, etc.).

Among a plurality of CSI-RSs and a plurality of IMRs configured to theUE, one CSI process may be defined in association with one CSI-RSresource for signal measurement and one interference measurementresource (IMR) for interference measurement. The UE feedbacks CSIinformation obtained via different CSI processes to a network (e.g., aBS) with independent periods and subframe offsets).

That is, each CSI process has independent CSI feedback configurations.The CSI-RS resource, the IMR resource association information, and theCSI feedback configuration may be indicated to the UE by a BS viahigher-layer signaling for each respective CSI process. For example, itis assumed that the UE may be configured by three CSI processes shown inTable 1 below.

TABLE 1 Signal Measurement CSI Process Resource (SMR) IMR CSI process 0CSI-RS 0 IMR 0 CSI process 1 CSI-RS 1 IMR 1 CSI process 2 CSI-RS 0 IMR 2

In Table 1 above, CSI-RS 0 and CSI-RS 1 are CSI-RS received from cell 1that is a serving cell of the UE and CSI-RS received from cell 2 as anadjacent cell that participates in cooperation, respectively. When it isassumed that IMR configured for each respective CSI process of Table 1above is configured as shown in Table 2 below,

TABLE 2 IMR eNB 1 eNB 2 IMR 0 Muting Data transmission IMR 1 Datatransmission Muting IMR 2 Muting Muting

with regard to IMR 0, cell 1 performs muting and cell 2 performs datatransmission, and the UE is configured to measure interference fromcells except for cell 1 based on IMR 0. Similarly, with regard to IMR 1,cell 2 performs muting and cell 1 performs data transmission, and the UEis configured to measure interference from cells except for cell 2 basedon IMR 1. In addition, with regard to IMR 2, both cells 1 and 2 performmuting, and the UE is configured to measure interference from cellsexcept for cells 1 and 2 based on IMR 2.

Accordingly, as shown in Tables 1 and 2 above, CSI information of CSIprocess 0 refers to optimum RI, PMI, and CQI information when data isreceived from cell 1. CSI information of CSI process 1 refers to optimumRI, PMI, and CQI when data is received from cell 2. CSI information ofCSI process 2 refers to optimum RI, PMI, and CQI information when datais received from cell 1 and interference is not generated from cell 2.

CSI processes configured to one UE may share dependent values for CoMPscheduling. For example, in case of joint transmission (JP) of cell 1and cell 2, when CSI process 1 in which a channel of cell 1 isconsidered as a signal part and CSI process 2 in which a channel of cell2 is considered as a signal part are configured to one UE, rank of CSIprocess 1 and CSI process 2 needs to be the same as a selected subbandindex in order to easily perform JT scheduling.

The period or pattern for transmitting a CSI-RS may be configured by aBS. In order to measure a CSI-RS, a UE needs to know CSI-RSconfiguration for each CSI-RS antenna port of a cell to which the UEbelongs. The CSI-RS configuration may include a downlink subframe indexfor transmitting a CSI-RS, a time-frequency location (e.g., the CSI-RSpatterns illustrated in FIGS. 8(a) to 8(e)) of a CSI-RS RE in atransmission subframe, and a CSI-RS sequence (which is a sequence usedas a CSI-RS and is pseudo-randomly generated according to apredetermined rule based on a slot number, a cell ID, a CP length,etc.). That is, a given BS may use a plurality of CSI-RS configurationsand may indicate CSI-RS configuration to be used for UE(s) in a cell ofthe plural CSI-RS configurations.

In addition, the CSI-RSs for the respective antenna ports need to bedifferentiated, and thus, resources for transmitting a CSI-RS for eachantenna port need to be orthogonal to each other. As described withreference to FIG. 8, CSI-RSs for the respective antenna ports may bemultiplexed using orthogonal frequency resources, orthogonal timeresources, and/or orthogonal code resources via FDM, TDM, and/or CDM.

Upon informing UEs in a cell of CSI-RS information (CSI-RSconfiguration), a BS needs to inform the UEs of information about timeand frequency to which a CSI-RS about each antenna port is mapped. Indetail, the information about time may contain subframe numbers fortransmitting the CSI-RS, a period for transmitting the CSI-RS, asubframe offset for transmitting the CSI-RS, an OFDM symbol number fortransmitting a CSI-RS RE of a specific antenna, etc. The informationabout frequency may contain frequency spacing for transmitting a CSI-RSRE of a specific antenna, an offset or shift value of an RE in afrequency axis, etc.

FIG. 9 illustrates an exemplary periodic CSI-RS transmission. A CSI-RSmay be transmitted periodically at every integer multiple of onesubframe (e.g. in every 5, 10, 20, 40 or 80 subframes).

Referring to FIG. 9, one radio frame is divided into 10 subframes,subframe 0 to subframe 9. The BS transmits a CSI-RS with a CSI-RStransmission period of 10 ms (i.e. in every 10 subframes) and a CSI-RStransmission offset of 3, by way of example. Different BSs may havedifferent CSI-RS transmission offsets so that CSI-RSs transmitted from aplurality of cells is uniformly distributed in time. If a CSI-RS istransmitted every 10 ms, its CSI-RS transmission offset may be one of 0to 9. Likewise, if the CSI-RS is transmitted every 5 ms, the CSI-RStransmission offset may be one of 0 to 4. If the CSI-RS is transmittedevery 20 ms, the CSI-RS transmission offset may be one of 0 to 19. Ifthe CSI-RS is transmitted every 40 ms, the CSI-RS transmission offsetmay be one of 0 to 39. If the CSI-RS is transmitted every 80 ms, theCSI-RS transmission offset may be one of 0 to 79. A CSI-RS transmissionoffset indicates a subframe in which a BS starts CSI-RS transmission inevery predetermined period. When the BS signals a CSI-RS transmissionperiod and offset to a UE, the UE may receive a CSI-RS from the BS insubframes determined by the CSI-RS transmission period and offset. TheUE may measure a channel using the received CSI-RS and thus may reportsuch information as a Channel Quality Indicator (CQI), a PMI, and/or aRank Indicator (RI) to the BS. Unless a CQI, a PMI and an RI areseparately described herein, they may be collectively referred to as aCQI (or CSI). A CSI-RS transmission period and offset may be setseparately for each individual CSI-RS configuration.

FIG. 10 illustrates an exemplary aperiodic CSI-RS transmission.Referring to FIG. 10, one radio frame is divided into 10 subframes,subframe 0 to subframe 9. Subframes carrying CSI-RSs may be indicated ina predetermined pattern. For instance, a CSI-RS transmission pattern maybe formed in units of 10 subframes and a 1-bit indicator may be set foreach subframe to indicate whether the subframe carries a CSI-RS. In theillustrated case of FIG. 10, the CSI-RS pattern tells that subframe 3and subframe 4 out of 10 subframes (i.e. subframe 0 to subframe 9) carryCSI-RSs. Such 1-bit indicators may be transmitted to a UE byhigher-layer signaling.

Various CSI-RS configurations are available as described above. Toenable a UE to receive CSI-RSs reliably for channel measurement, an eNBneeds to signal a CSI-RS configuration to the UE. Now a description willbe given below of embodiments of the present invention for signaling aCSI-RS configuration to a UE.

CSI-RS Configuration Signaling

In general, the eNB may signal a CSI-RS configuration to the UE in twomethods.

One of the methods is for the eNB to broadcast CSI-RS configurationinformation to UEs by dynamic broadcast channel (DBCH) signaling.

In the legacy LTE system, an eNB may transmit system information to UEson a broadcast channel (BCH). If the system information is too much tobe transmitted on the BCH, the eNB may transmit the system informationin the same manner as downlink data transmission. Notably, the eNB maymask the CRC of a PDCCH associated with the system information by anSI-RNTI, instead of a particular UE ID. Thus, the system information istransmitted on a PDSCH like unicast data. All UEs within the cell maydecode the PDCCH using the SI-RNTI and thus acquire the systeminformation by decoding the PDSCH indicated by the PDCCH. Thisbroadcasting scheme may be referred to as DBCH signaling,distinguishably from general Physical BCH (PBCH) signaling.

Two types of system information are usually broadcast in the legacy LTEsystem. One type of system information is a Master Information Block(MIB) transmitted on a PBCH and the other type of system information isa system information block (SIB) multiplexed with general unicast datain a PDSCH region. As the legacy LTE system defines SIB type 1 to SIBType 8 (SIB1 to SIB8) for system information transmission, a new SIBtype may be defined for CSI-RS configuration information which is newsystem information not defined as any conventional SIB type. Forexample, SIB9 or SIB10 may be defined and the eNB may transmit CSI-RSconfiguration information to UEs within its cell in SIB9 or SIB10 byDBCH signaling.

The other method for signaling CSI-RS configuration information is thatthe BS transmits CSI-RS configuration information to each UE by radioresource control (RRC) signaling. That is, the CSI-RS configurationinformation may be provided to each UE within the cell by dedicated RRCsignaling. For example, while a UE is establishing a connection to theBS during initial access or handover, the BS may transmit the CSI-RSconfiguration information to the UE by RRC signaling. Alternatively oradditionally, the BS may signal the CSI-RS configuration information tothe UE in an RRC signaling message requesting a channel state feedbackbased on CSI-RS measurement to the UE.

CSI-RS Configuration Indication

A random BS may use a plurality of CSI-RS configurations and transmit aCSI-RS according to each CSI-RS configuration to a UE on a predeterminedsubframe. In this case, the BS may inform the UE of the plural CSI-RSconfigurations and inform the UE of a CSI-RS to be used for channelstate measurement for Channel Quality Information (CQI) or Channel StateInformation (CSI) feedback among the plurality of CSI-RS configurations.

A description will be given of indication of a selected CSI-RSconfiguration and CSI-RSs to be used for channel measurement to a UE bya BS according to an embodiment of the present invention.

FIG. 11 illustrates an example of using two CSI-RS configurations.Referring to FIG. 11, one radio frame is divided into 10 subframes,subframe 0 to subframe 9. For a first CSI-RS configuration (CSI-RS1), aCSI-RS transmission period is 10 ms and a CSI-RS transmission offset is3. For a second CSI-RS configuration (CSI-RS2), a CSI-RS transmissionperiod is 10 ms and a CSI-RS transmission offset is 4. The BS may signalthe two CSI-RS configurations to the UE and notify the UE of a CSI-RSconfiguration to be used for CQI (or CSI) feedback.

Upon receipt of a CQI feedback request for a specific CSI-RSconfiguration from the BS, the UE may measure a channel state using onlyCSI-RSs having the specific CSI-RS configuration. To be more specific, achannel state is a function of a CSI-RS reception quality, the amount ofnoise/interference, and a correlation coefficient between them. TheCSI-RS reception quality may be measured using only the CSI-RSs havingthe specific CSI-RS configuration, and the amount of noise/interferenceand the correlation coefficient (e.g. an interference covariance matrixrepresenting the direction of interference) may be measured in asubframe carrying the CSI-RSs or a predetermined subframe. For example,in FIG. 11, when the BS requests a feedback for the first CSI-RSconfiguration to the UE, the UE may measure a reception quality usingCSI-RSs received in a fourth subframe (subframe index 3) in a radioframe. For the UE to calculate the amount of noise/interference and thecorrelation coefficient, the BS may indicate an odd-numbered subframe tothe UE. Alternatively or additionally, the BS may confine the UE to aspecific single subframe (e.g. subframe index 3), for measuring theCSI-RS reception quality and calculating the amount ofnoise/interference and the correlation coefficient.

For instance, the CSI-RS reception quality may be theSignal-to-Interference plus Noise Ratio (SINR) of the CSI-RSs, expressedas S/(I+N) (S is the strength of the received signal, I is the amount ofinterference, and N is the amount of noise). The strength of thereceived signal, S may be measured using CSI-RSs in a subframe carryingthe CSI-RSs as well as a signal for the UE. Since I and N vary accordingto the amount of interference from adjacent cells and the directions ofsignals from the adjacent cells, they may be measured using CRSstransmitted in a subframe designated for measuring S, or in a separatelydefined subframe.

The amount of noise/interference and the correlation coefficient may bemeasured on REs carrying CRSs or CSI-RSs in a subframe or on null REsdesignated to facilitate noise/interference measurement. To measurenoise/interference on CRS REs or CSI-RS REs, the UE may first recoverCRSs or CSI-RSs, acquire a noise and interference signal by subtractingthe recovered CRSs or CSI-RSs from a received signal, and thus calculatea statistical noise/interference value. A null RE is an empty RE withzero transmission power, carrying no signal. Null REs facilitatemeasurement of a signal transmitted from a BS other than the BS. Whileall of CRS REs, CSI-RS REs, and null REs may be used to calculate theamount of noise/interference and the correlation coefficient, the BS maydesignate specific REs for noise/interference measurement for the UE,among the above REs. This is because appropriate REs need to be set formeasurement at the UE depending on a neighbor cell transmits a datasignal or a control signal on the REs. The neighbor cell may transmit adata signal or a control signal on the REs according to synchronizationor non-synchronization between cells, a CRS configuration, and a CSI-RSconfiguration. Therefore, the BS may determine the synchronization ornon-synchronization between cells, the CRS configuration, and the CSI-RSconfiguration and designate REs for measurement for the UE according tothe determination. That is, the BS may indicate to the UE that the UEwill measure noise/interference using all or part of the CRS REs, CSI-RSREs and null REs.

For example, a plurality of CSI-RS configurations is available to theBS. The BS may indicate one or more CSI-RS configurations, and mayindicate to the UE a CSI-RS configuration selected for CQI feedback fromamong the CSI-RS configurations and the positions of null REs, for CSIfeedback. The CSI-RS configuration selected for CQI feedback may be aCSI-RS configuration with non-zero transmission power, relative to nullREs with zero transmission power. For example, the BS may indicate oneCSI-RS configuration for channel measurement to the UE and the UE mayassume that CSI-RSs are transmitted with non-zero transmission power inthe CSI-RS configuration. Additionally, the BS may indicate a CSI-RSconfiguration with zero transmission power (i.e. the positions of nullREs) to the UE and the UE may assume that the REs of the CSI-RSconfiguration have non-zero power. In other words, the BS may notify theUE of a CSI-RS configuration with non-zero transmission power and, inthe presence of a CSI-RS configuration with zero transmission power, theBS may indicate the positions of null REs in the CSI-RS configurationwith zero transmission power to the UE.

As a modification example to the above-described CSI-RS configurationindication method, the BS may signal a plurality of CSI-RSconfigurations to the UE and may also signal all or part of the CSI-RSconfigurations, selected for CQI feedback to the UE. Upon receipt of aCQI feedback for a plurality of CSI-RS configurations, the UE maymeasure CQIs using CSI-RSs corresponding to the CSI-RS configurationsand report the CQIs to the BS.

To allow the UE to transmit the CQIs for the respective CSI-RSconfigurations, the BS may predefine uplink resources for CQItransmission for each CSI-RS configuration and preliminarily provideinformation about the uplink resources to the UE by RRC signaling.

Additionally, the BS may dynamically trigger CQI transmission for aCSI-RS configuration to the UE. The dynamic triggering of CQItransmission may be carried out through a PDCCH. The PDCCH may indicatea CSI-RS configuration for CQI measurement to the UE. Upon receipt ofthe PDCCH, the UE may feedback a CQI measurement result for the CSI-RSconfiguration indicated by the PDCCH to the BS.

CSI-RSs may be set to be transmitted in different subframes or in thesame subframe in a plurality of CSI-RS configurations. When CSI-RSshaving different CSI-RS configurations are transmitted in the samesubframe, it is necessary to distinguish them. To identify the CSI-RSshaving different CSI-RS configurations in the same subframe, one or moreof CSI-RS time resources, frequency resources, and code resources may bedifferent for them. For example, the positions of REs carrying CSI-RSsmay be different for different CSI-RS configurations in time or infrequency (for example, CSI-RSs with a CSI-RS configuration aretransmitted on REs illustrated in FIG. 8(a) in a subframe and CSI-RSswith another CSI-RS configuration are transmitted on REs illustrated inFIG. 8(b) in the same subframe) (classification according to time andfrequency resources). When CSI-RSs with different CSI-RS configurationsare transmitted on the same RE, different CSI-RS scrambling codes may beapplied to the CSI-RSs (classification according to code resources).

Quasi Co-Located (QC)

A UE may receive data from all transmission points (TPs) that belongs tocoordinated multi-point (CoMP), for example, TP1 and TP2 and thustransmit CSI about all TPs that belong to the CoMP set. In this case,RSs may also be transmitted to the UE from a plurality of TPs in theCoMP set. In this case, when characteristics for channel estimation fromdifferent RS ports of different TPs can be shared, load and complexityof reception processing of the UE may be reduced. In addition, whencharacteristics for channel estimation from different RS ports of thesame TP can be shared, load and complexity of reception processing ofthe UE may be reduced. Accordingly, an LTE-A system has proposed amethod for sharing characteristics for channel estimation between RSports.

For channel estimation between RS ports, the LTE-A system has introducedthe concept “quasi co-located (QCL)”. For example, when the large-scaleproperty of a radio channel for transmitting a symbol through oneantenna port can be inferred from a radio channel for transmitting asymbol through another antenna port, it may be expressed that the twoantenna ports are quasi co-located (QCL). Here, the large-scale propertyincludes one or more of delay spread, Doppler spread, Doppler shift,average gain, and average delay. Hereinafter, the quasi co-located willbe referred to as QCL.

That is, when two antenna ports are QCL, it means that the large-scaleproperty of a radio channel from one antenna port is the same as thelarge-scale property of a radio channel from the other antenna port.When different types of RSs are QCL, the large-scale property of a radiochannel from one type of antenna port can be replaced with thelarge-scale property of a radio channel from another type of antennaport.

According to the concept of the QCL, with regard to non-QCL antennaports, the UE cannot assume the same large-scale property between radiochannels from the corresponding antenna ports. That is, in this case,the UE needs to perform independent processing for respective non-QCLantenna ports configured for timing acquisition and tracking, frequencyoffset estimation and compensation, Doppler estimation, etc.

It is advantageous in that the UE can perform the following operationbetween antenna ports that can assume QCL. First, the UE can use resultsof delay spread, Doppler spectrum, and Doppler spread estimation of aradio channel from one antenna port upon estimation of a channel for aradio channel from another antenna port. Then, with respect to frequencyshift and reception timing, the UE may perform time and frequencysynchronization on one antenna port and then apply the samesynchronization on demodulation of another antenna port. Then, withrespect to average reception power, the UE can level measurement ofreference signal received power (RSRP) of two or more antenna ports.

Upon reception of DM RS-based downlink-associated DCI format through acontrol channel (PDCCH or ePDCCH), the UE performs channel estimation onthe corresponding PDSCH through a DM RS sequence and then performs datademodulation. For example, when the UE can assume configuration of a DMRS port received from downlink scheduling grant to be QCL with a CRSport, the UE can apply the same value as an estimated value of thelarge-scale property of a radio channel, estimated from a CRS, uponestimation of a channel through the corresponding DM RS port. This isbecause a CRS is an RS that is broadcast with relatively high densityevery subframe and all bands, and thus, an estimated value of thelarge-scale property can be more stably acquired from the CRS, ingeneral. On the other hand, since a DM RS is transmit UE-specificallywith respect to a specific scheduled RB, and a precoding matrix used fortransmission in PRG units by a BS varies, an effective channel receivedby the UE may vary in PRG units. Accordingly, when the DM RS is used forestimation of the large-scale property of a radio channel over awideband, performance may be degraded. Since a CSI-RS has a relativelylong transmission period and low density, when the CSI-RS is used forestimation of the large-scale property of the radio channel, performancemay also be degraded.

That is, QCL assumption between antenna ports can be applied toreception of various downlink RSs, channel estimation, channel statereporting, etc.

PRB Bundline

PRB bundling refers to application of the same PMI across a pluralityadjacent resource blocks during data transmission. In other words, inorder to perform PMI/RI reporting, a UE assumes a plurality of resourceblocks in the frequency domain as one granularity for precoding.

A system bandwidth is divided into fixed system bandwidths according toprecoding resource block group (PRG) size P′, and each PRG includesconsecutive PRBs. If N_(RB) ^(DL) mod P′>0, the size of one of PRGs isN_(RB) ^(DL)−P′└N_(RB) ^(DL)/P′┘.

As shown in Table 3 below, a PRG size assumed by the UE in an LTE systemwill be described in more detail. In the LTE system, the PRB sizeassumed by the UE with respect to a given system bandwidth is defined asshown in Table 3 below.

TABLE 3 System Bandwidth (N_(RB) ^(DL)) PRG Size (P′) (PRBs) ≦10 1 11-262 27-63 3  64-110 2

For example, when a configured downlink bandwidth (N_(RB) ^(DL)) is 25,the PRG size is defined as 2 as shown in Table 2 above. Accordingly,during PRB bundling, one of PRGs includes one PRB. That is, a systembandwidth is divided into 13 of total PRGs: 12 PRGs including two PRBs;and one PRG including one PRB. In this case, the UE may assume that thesame precoder to all scheduled PRBs that belong to one PRG.

PRB Bundling in CoMP System

A legacy LTE system does not support CoMP between BSs, and thus, each UEhas one CSI process. Thus, the UE and the BS can obviously determinewhether PRB bundling is applied according to whether PMI/RI of the CSIprocess is reported.

For example, in the legacy LTE system, when the PMI/RI reporting of theCSI process is enabled, PRB bundling can be applied according to systembandwidth. When the PRB bundling is applied, the UE considers channelsof adjacent PRBs to which the same precoding is applied, as the samechannel, and performs interpolation across adjacent PRBs to which thebundling is applied. As a result, channel estimation performance anddemodulation performance can be enhanced. On the other hand, whendownlink scheduling is performed using channel reciprocity in a TDDsystem, the BS can know some properties of a downlink channel fromuplink SRS and thus does not have to receive PMI and RI as feedback fromthe UE. Thus, in this case, it is effective to disable PMI/RI report. Inthis case, the BS does not apply PRB bundling and estimates a downlinkchannel in PRB units to configure appropriate PMI and RI.

However, in a CoMP system, the UE needs to feedback channel informationof a transmission point or an adjacent cell that participates in CoMP aswell as channel information of a serving cell (or a transmission point),to the BS. That is, in the CoMP system, the UE feedbacks CSI accordingto multiple CSI processes instead of a single CSI process.

Multiple CSI processes configured for one UE may be independentlyconfigured. For example, with regard to each CSI process, PMI/RIreporting may be independently enabled or disabled by higher-layersignaling (e.g., RRC signaling). In addition, when a TDD system performsCoMP, a cell receiving weak uplink interference among cells thatparticipates in CoMP can acquire PMI/RI information using channelreciprocity due to different uplink interference environments for therespective cells, but a cell receiving strong uplink interference amongthe cells cannot acquire PMI/RI information using the channelreciprocity.

Accordingly, it is effective to enable or disable PMI/RI reportingaccording to whether channel reciprocity is used for each respective CSIprocess. That is, with regard to some CSI processes among multiple CSIprocesses, PMI/RI reporting may be enabled, and with regard to theremaining CSI processes, PMI/RI reporting may be disabled.

In the CoMP system, each CSI process of the multiple CSI processes canhave independent PMI/RI reporting configuration, a problem arises interms of whether PRB bundling is applied.

Hereinafter, when each CSI process of the multiple CSI processes hasindependent PMI/RI reporting configuration, a method for determiningwhether PRB bundling is applied will be proposed. That is, operationsaccording to the following embodiments of the present invention may bepromised between the BS and the UE, thereby preventing ambiguity aboutwhether PRB bundling is applied.

Embodiment 1-1

According to Embodiment 1-1 of the present invention, when PMI/RIreporting of at least one CSI process among CSI processes configured forone serving cell is enabled, a UE assumes that PRB bundling is appliedto DM RS and performs demodulation upon reception of data from a BS.That is, when PMI/RI reporting of at least one CSI process amongmultiple CSI processes configured for one serving cell is enabled, theBS applies PRB bundling to data and DM RS during data transmission. TheUE can perform channel interpolation using the DM RS in PRB group (PRG)units to which the bundling is applied and can more accurately estimatea channel. Then, the UE performs data demodulation based on theestimated channel.

Similarly, when PMI/RI reporting of all CSI processes configured for oneserving cell is disabled, the UE assumes that PRB bundling is notapplied to DM RS and performs demodulation upon reception of data from aBS. That is, when PMI/RI reporting of all CSI processes configured forone serving cell is disabled, the BS does not apply PRB bundling to dataand DM RS during data transmission. The UE performs channel estimationfor each respective PRB using DM RS. Then, the UE performs datademodulation based on the estimated channel.

Embodiment 1-2

According to Embodiment 1-2 of the present invention, when PMI/RIreporting of at least one CSI process among CSI processes configured forone serving cell is disabled, a UE assumes that PRB bundling is notapplied to DM RS and performs demodulation upon reception of data from aBS. That is, when PMI/RI reporting of at least one CSI process amongmultiple CSI processes configured for one serving cell is disabled, theBS does not apply PRB bundling to data and DM RS during datatransmission. The UE performs channel estimation using DM RS for eachrespective PRB. Then, the UE performs data demodulation based on theestimated channel.

Similarly, when PMI/RI reporting of all CSI processes configured for oneserving cell is enabled, the UE assumes that PRB bundling is applied toDM RS and performs demodulation upon reception of data from a BS. Thatis, when PMI/RI reporting of all CSI processes configured for oneserving cell is enabled, the BS applies PRB bundling to data and DM RSduring data transmission. The UE performs channel interpolation using DMRS in PRB group (PRG) units to which the bundling is applied and canmore accurately estimate a channel. Then, the UE performs datademodulation based on the estimated channel.

FIG. 12 is a flowchart of a channel estimation method according toEmbodiment 1-2 of the present invention.

First, a UE receives information about at least one channel stateinformation (CSI) configuration for reporting CSI of a serving cell(S1201). A detailed description of the CSI configuration is the same asthe above configuration, and thus, will be omitted here.

Then, when all of at least one CSI configuration configured for aserving cell is configured to report PMI/RI, the UE determines that thesame precoding matrix is applied to a plurality of resource blocks, andwhen CSI configuration that is configured not to report PMI/RI among atleast one CSI configuration is present, the UE determines that aprecoding matrix is applied to one resource block (S1203).

That is, when PMI/RI reporting of at least one CSI process of multipleCSI processes configured for one serving cell is disabled, the BS doesnot apply PRB bundling to data and DM RS during data transmission. WhenPMI/RI reporting of all configured CSI processes is enabled, the UEassumes that PRB bundling is applied to DM RS and performs demodulationupon reception of data from the BS.

Then, the UE receives DM RS from the BS (S1205). In this case, the UEmay receive DM RS in consideration of whether PRB bundling is applied.When PMI/RI reporting of at least one CSI process of multiple CSIprocesses for a serving cell is disabled, the UE determines that PRBbundling is not applied and receives DM RS. When PMI/RI reporting of allCSI processes configured for a serving cell is enabled, the UEdetermines that PRB bundling is applied and receives DM RS.

Then, the UE estimates a channel based on a result of the aforementioneddetermination step (S1207). The UE may receive DM RS based on whetherPRB bundling is applied, estimate a channel using the received DM RS,and demodulate data.

Embodiment 1-3

According to Embodiment 1-1 or 1-2, whether PRB bundling is applied issemi-statically determined according to configuration of multiple CSIprocesses. Embodiment 1-3 proposes a method in which a UE dynamicallydetermines whether PRB bundling is applied using QCL informationtransmitted through DCI. In addition, the UE may add a new field to DCIother than QCL information to indicate whether PRB bundling is applied.

In a QCL environment, the BS signals QCL information about a cell (or atransmission point) from which DM RS is transmitted, to the UE using aDCI field such that the UE appropriately performs data demodulation.That is, the BS may signal a channel corresponding to a CSI processamong configured multiple CSI processes, through which DM RS istransmitted, to the UE using a specific field of DCI. For example, whenthe BS may add a 2-bit field to DCI and the corresponding bits is 00,01, and 10, the bits may indicate a first CSI process, a second CSIprocess, and a third CSI process, respectively, to indicate QCLinformation. That is, in the case of 00, the UE assumes that thereceived DM RS is transmitted from a channel corresponding to the firstCSI process and performs data demodulation.

That is, in a QCL environment, the UE may determine whether PRB bundlingis applied to DM RS based on QCL information (i.e., CSI processinformation) received through DCI.

When PMI/RI reporting of the CSI process signaled to the UE by the BSusing QCL information is disabled, the UE assumes that PRB bundling isnot applied to DM RS and performs demodulation during data reception.That is, when PMI/RI reporting of the CSI process signaled to the UE bythe BS using QCL information is disabled, the BS does not apply PRBbundling to data and DM RS during data transmission. The UE performschannel estimation using DM RS for each respective PRB. Then, the UEperforms data demodulation based on the estimated channel.

Similarly, when PMI/RI reporting of the CSI process signaled to the UEby the BS using QCL information is enabled, the UE assumes that PRBbundling is applied to DM RS and performs demodulation during datareception. That is, when PMI/RI reporting of the CSI process signaled tothe UE by the BS using QCL information is enabled, the BS applies PRBbundling to data and DM RS during data transmission. The UE performschannel estimation using DM RS in PRB group units (e.g., in PRG units)to which bundling is applied and can more accurately estimate a channel.Then, the UE performs data demodulation based on the estimated channel.

Embodiment 1-4

When N or more CSI process, PMI/RI reporting of which is enabled, amongCSI processes configured for one serving cell, a UE assumes that PRBbundling is applied to DM RS and performs demodulation upon reception ofdata from a BS. That is, when N or more CSI process, PMI/RI reporting ofwhich is enabled, among CSI processes configured for one serving cell,the BS applies PRB bundling to data and DM RS during data transmission.The UE can perform channel interpolation using the DM RS in PRB group(e.g., PRG) units to which the bundling is applied and can moreaccurately estimate a channel. Then, the UE performs data demodulationbased on the estimated channel. N is a static value or issemi-statically determined by RRC signaling, etc.

Hereinafter, a method for determining whether PRB bundling is appliedwhen whether PMI/RI is reported is independently configured for eachrespective multiple non-zero-power (NZP) CSI-RS will be proposed. Sinceonly a cell corresponding to a specific CSI process transmits data in aCoMP system, all configured CSI processes may not be considered.Accordingly, all configured CSI processes are not considered and whetherPRB bundling is applied may be determined with respect to only a CSIprocess corresponding to a cell that actually transmits data based onNZP CSI-RS.

Embodiment 2-1

When PMI/RI reporting is enabled with respect to at least one NZP CSI-RSamong configured NZP CSI-RSs, a UE assumes that PRB bundling is appliedto DM RS and performs demodulation upon reception of data from a BS.That is, when PMI/RI reporting is enabled with respect to at least oneNZP CSI-RS among NZP CSI-RSs configured for one UE, the BS applies PRBbundling to data and DM RS during data transmission. The UE can performchannel interpolation using the DM RS in PRB group (e.g., PRG) units towhich the bundling is applied and can more accurately estimate achannel. Then, the UE performs data demodulation based on the estimatedchannel.

Similarly, when PMI/RI reporting is disabled with respect to allconfigured NZP CSI-RSs, the UE assumes that PRB bundling is not appliedto DM RS and performs demodulation upon reception of data from the BS.That is, when PMI/RI reporting is disabled with respect to all NZPCSI-RSs configured for one UE, the BS does not apply PRB bundling todata and DM RS during data transmission. The UE performs channelestimation using DM RS for each respective PRB. Then, the UE performsdata demodulation based on the estimated channel.

Embodiment 2-2

When PMI/RI reporting is disabled with respect to at least one NZPCSI-RS among configured NZP CSI-RSs, a UE assumes that PRB bundling isnot applied to DM RS and performs demodulation upon reception of datafrom a BS. That is, when PMI/RI reporting is disabled with respect to atleast one NZP CSI-RS among NZP CSI-RSs configured for one UE, the BSdoes not apply PRB bundling to data and DM RS during data transmission.The UE performs channel estimation using DM RS for each respective PRB.Then, the UE performs data demodulation based on the estimated channel.

Similarly, when PMI/RI reporting is enabled with respect to allconfigured NZP CSI-RSs, the UE does not assume that PRB bundling isapplied to DM RS and performs demodulation upon reception of data from aBS. That is, when PMI/RI reporting is enabled with respect to all NZPCSI-RSs configured for one UE, the BS applies PRB bundling to data andDM RS during data transmission. The UE can perform channel interpolationusing the DM RS in PRB group (e.g., PRG) units to which the bundling isapplied and can more accurately estimate a channel. Then, the UEperforms data demodulation based on the estimated channel.

Embodiment 2-3

According to Embodiment 2-1 or 2-2, whether PRB bundling is applied issemi-statically determined according to configuration of NZP CSI-RSconfiguration. Embodiment 2-3 proposes a method in which a UEdynamically determines whether PRB bundling is applied using QCLinformation transmitted through DCI.

In a QCL environment, the BS signals QCL information about atransmission point from which DM RS is transmitted, to the UE using aDCI field such that the UE appropriately performs data demodulation.That is, the BS may signal a channel for transmitting NZP CSI-RS amongconfigured multiple NZP CSI-RSs, through which DM RS is transmitted, tothe UE using a specific field of DCI. For example, when the BS may add a2-bit field to DCI and the corresponding bits is 00, 01, and 10, thebits may indicate a first NZP CSI-RS, a second NZP CSI-RS, and a thirdNZP CSI-RS, respectively, to indicate QCL information. That is, in thecase of 00, the UE assumes that the received DM RS is transmitted from achannel through which the first NZP CSI-RS is transmitted and performsdata demodulation.

That is, in a QCL environment, the UE may determine whether PRB bundlingis applied to DM RS based on QCL information (i.e., NZP CSI-RSinformation) received through DCI.

When PMI/RI reporting of the NZP CSI-RS signaled to the UE by the BSusing QCL information is disabled, the UE assumes that PRB bundling isnot applied to DM RS and performs demodulation during data reception.That is, when PMI/RI reporting of the NZP CSI-RS signaled to the UE bythe BS using QCL information is disabled, the BS does not apply PRBbundling to data and DM RS during data transmission. The UE performschannel estimation using DM RS for each respective PRB. Then, the UEperforms data demodulation based on the estimated channel.

Similarly, when PMI/RI reporting of the NZP CSI-RS signaled to the UE bythe BS using QCL information is enabled, the UE assumes that PRBbundling is applied to DM RS and performs demodulation during datareception. That is, when PMI/RI reporting of the NZP CSI-RS signaled tothe UE by the BS using QCL information is enabled, the BS applies PRBbundling to data and DM RS during data transmission. The UE performschannel estimation using DM RS in PRB group units (e.g., in PRG units)to which bundling is applied and can more accurately estimate a channel.Then, the UE performs data demodulation based on the estimated channel.

Embodiment 3-1

According to Embodiment 1-1, 1-2, or 1-4, whether PRB bundling isapplied is determined based on all CSI processes configured for onecell. That is, when the standard of Embodiment 1-1, 1-2, or 1-4 isapplied to all CSI processes configured for one cell and is satisfied,PRB bundling is applied.

However, CSI processes corresponding to actual data transmission to a UEare no more than some of the all CSI processes. For example, when threeCSI processes are configured for one serving cell as shown in Tables 1and 2 above, if cell 2 as an adjacent cell transmits data through DPS, aCSI process corresponding to actual data transmission is only CSIprocess 2. Accordingly, it is ineffective to apply the standard for PRBbundling to all CSI processes.

For a more effective operation, the present invention proposes reductionin objects of CSI processes to which the standard of PRB bundling isapplied using QCL information transmitted through PQI. For example, a UEacquires NZP CSI-RS information of a cell that participates in actualdata transmission through PQI and applies the standard of PRB bundlingin consideration of only a process using the NZP CSI-RS among CSIprocesses configured for the UE. That is, according to theaforementioned embodiment, when NZP CSI-RS determined as PQI is NZPCSI-RS 1, the UE determines whether PRB bundling is applied with respectto CSI process 1 in consideration of Embodiments 1-1, 1-2, or 1-4. Inaddition, according to the aforementioned embodiment, when NZP CSI-RSdetermined as PQI is NZP CSI-RS 0, the UE determines whether PRBbundling is applied with respect to CSI processes 0 and 2 inconsideration of Embodiments 1-1, 1-2, or 1-4.

When an LTE release-11 UE receives DCI format 1A in a transmission mode10, PQI information is first state information among four states of PQIdefined in DCI format 2D. Thus, in this case, a CSI process using NZPCSI-RS corresponding to the first PQI state is selected and the standardof PRB bundling of Embodiment 1-1, 1-2, or 1-4 is applied inconsideration of the CSI process.

Embodiment 3-2

In Embodiment 3-1, QCL behavior B (QCLBB) defined in LTE release-11operates without any problem. This is because, in the case of QCLBB, aUE can obtain NZP CSI-RS information corresponding to an actual datatransmission channel from PQI information. However, in the case of QCLbehavior A (QCLBA) that assumes QCL between serving cell CRS, and NZPCSI-RS and DM RS (or between all ports), PQI information does notcontain NZP CSI-RS information. Accordingly, an operation of Embodiment3-1 needs to be corrected in consideration of this fact.

LTE release-11 basically assumes partial QCL between NZP CSI-RS and CRS.As described above, the QCLBB assumes partial QCL between NZP CSI-RS andCRS. In this case, CRS and partial QCL NZP CSI-RS are signal to the UEby RRC signaling. In other words, CRSs of a specific cell is mapped torespective NZP CSI-RS configurations, and the UE assumes QCL between NZPCSI-RS and CRS with reference to a mapping table. According to thepresent invention, for convenience of description, the mapping table isreferred to as “CRS to NZP CSI-RS QCL mapping table”.

The partial QCL refers to partial QCL information among various piecesof QCL information and refers to only {Doppler spread and Doppler shift}in behavior B. On the other hand, the QCLBA assumes QCL with respect tonumerous information including the partial QCL, that is, {(Doppler shiftand Doppler spread), average delay, and delay spread)} and assumes QCLbetween all CRS, NZP CSI-RS, and DM RS ports configured for the UE.

Thus, when the UE is configured to QCLBA, the UE first assumes QCLbetween DM RS and serving cell CRS (for example, because QCLBA can beinterpreted as a case in which all ports are QCL). In this case, NZPCSI-RS mapped to serving-cell CRS can be confirmed using the CRS to NZPCSI-RS QCL mapping table received by RRC signaling for QCLBB. In thepresent invention, for convenience of description, NZP CSI-RS isreferred to as “standard NZP CSI-RS”.

In Embodiment 3-1, NZP CSI-RS information of a cell that participates inactual data transmission through PQI and a CSI process to which PRBbundling standard is to be applied is selected using the NZP CSI-RS.However, on the other hand, in Embodiment 3-2, a CSI process to whichthe PRB bundling standard is to be applied is selected using thestandard NZP CSI-RS. For example, the UE acquires that standard NZPCSI-RS information through the aforementioned process and applies thePRB bundling standard in consideration of only a process using the NZPCSI-RS among CSI processes configured to the UE. That is, in theexamples of Tables 1 and 2, when the standard NZP CSI-RS is NZP CSI-RS1, the UE determines whether PRB bundling is applied with respect to CSIprocess 1 in consideration of Embodiment 1-1, 1-2, or 1-4. In addition,in the examples of Tables 1 and 2, when the standard NZP CSI-RS is NZPCSI-RS 0, the UE determines whether PRB bundling is applied withreference to CSI processes 0 and 2 in consideration of Embodiment 1-1,1-2, or 1-4.

In CoMP scenario 4, adjacent cells share the same cell ID, and thus, oneserving cell CRS are mapped to various NZP CSI-RSs in the CRS to NZPCSI-RS QCL mapping table. In this case, Embodiment 3-2 has variousstandard NZP CSI-RSs. Likewise, when various standard NZP CSI-RSs arepresent, Embodiment 3-2 can be extensively applied. That is, the UErecognizes CSI processes using one NZP CSI-RS among standard NZP CSI-RSsand determines whether PRB bundling is applied with respect to the CSIprocesses using Embodiment 1-1, 1-2, or 1-4.

In the case of QCLBA, when the UE is not configured by the CRS to NZPCSI-RS QCL mapping table, Embodiment 1-1, 1-2, or 1-4 is used.

Embodiment 4

More briefly, when multiple CSI processes are configured to a UE, the UEassumes that PRB bundling is not applied to DM RS and performsdemodulation upon reception of data from a BS. That is, when themultiple CSI processes are configured to one UE, the BS does not applydata and DM RS during data transmission. The UE performs channelestimation using DM RS for each respective PRB. Then, the UE performsdata demodulation based on the estimated channel.

Embodiment 5

More briefly, when multiple CSI processes are configured to a UE, the UEassumes that PRB bundling is applied to DM RS and performs demodulationupon reception of data from a BS. That is, when the multiple CSIprocesses are configured to one UE, the BS applies data and DM RS duringdata transmission. The UE can perform channel interpolation using the DMRS in PRB group (PRG) units to which the bundling is applied and canmore accurately estimate a channel. Then, the UE performs datademodulation based on the estimated channel.

Different methods of the aforementioned embodiments can be appliedaccording to QCL behaviors. For example, in the case of QCLBA,Embodiment 4 or 5 may be used, and in the case of QCLBB, Embodiment 3-1may be used.

Embodiment 6

When multiple CSI processes are configured for a UE, the UE determineswhether PRB bundling is applied based on a CSI process with a lowestindex. That is, when the multiple CSI processes are configured for oneUE, if RI/PMI reporting is enabled with respect to the CSI process witha lowest index, a BS applies PRB bundling to data and PRB bundlingduring data transmission.

When RI/PMI reporting is enabled with respect to the CSI process with alowest index, the UE assumes that PRB bundling is applied to DM RS andperforms demodulation upon reception of data from the BS. The UE mayperform channel interpolation using DM RS in PRB group units (e.g., inPRG units) to which bundling is applied and can more accurately estimatea channel. Then, the UE performs data demodulation based on theestimated channel.

When RI/PMI reporting is disabled with respect to the CSI process with alowest index, the UE does not apply bundling.

Embodiment 7

In frequency selective DPS of a CoMP operation, one UE receives datafrom cell A of a specific PRB in a specific subframe and receives datafrom cell B of another PRB. When PRBs that receive data from cell A andcell B are referred to as PRB-A and PRB-B, respectively, the UE shouldnot group PRB-A and PRB-B to one PRB bundle. This is because PRB-A andPRB-B have different reception channels due to different transmissionchannels.

Accordingly, in order to prevent faulty operations of PRB bundling inthe frequency selective DPS, a BS asks the UE not to apply PRB bundlingin the corresponding subframe by dynamic signaling such as DCI duringthe frequency selective DPS. In addition, when the UE can know whetherthe frequency selective DPS is applied, the UE does not expect PRBbundling between PRB-A and PRB-B during data reception through thefrequency selective DPS.

Embodiment 8

More briefly, whether PRB bundling is applied may be semi-staticallysignaled to a UE by RRC signaling. When PRB bundling is enabled by RRCsignaling, the UE performs PRB bundling, and otherwise, the UE does notperform PRB bundling. In an LTE release-10 system, even if whether PRBbundling is applied is not signaled by RRC signaling, multiple CSIprocesses are configured in release-11, and thus whether PRB bundling isapplied may be signaled by RRC signaling, which is one solution. An LTErelease-11 UE defines whether PRB bundling is applied in different waysaccording to a transmission mode. For example, in transmission mode 10,whether bundling is applied is determined by the above proposed RRCsignaling. In transmission mode 9, whether bundling is applied isdetermined using a conventional method. Here, a new field may be addedto DCI instead of RRC signaling to signal whether PRB bundling isapplied.

In addition, Embodiment 8 may be combined with the aforementionedembodiments. For example, when Embodiment 8 is combined with Embodiment1-2 above, if PMI/RI reporting of all CSI processes is enabled, the UEperforms PRB bundling, and if PMI/RI reporting of all CSI processes isdisabled, the UE does not perform PRB bundling. In addition, when a CSIprocess, PMI/RI reporting of which is disabled, and a CSI process,PMI/RI reporting of which is enabled, are combined, whether PRB bundlingis applied is determined by the RRC signaling.

As another example, when Embodiment 3-1 above and Embodiment 1-2 aboveare combined, the UE selects a CSI process using PQI information in amanner of Embodiment 3-1 above. Then, the UE applies Embodiment 1-2 tothe selected CSI process. In this case, when PMI/RI reporting of the allselected CSI processes is enabled, the UE performs PRB bundling, andwhen PMI/RI reporting of the all selected CSI processes is disabled, theUE does not perform PRB bundling. In addition, when a CSI process,PMI/RI reporting of which is disabled, and a CSI process, PMI/RIreporting of which is enabled, are combined, whether PRB bundling isapplied is determined by the RRC signaling.

Alternatively, Embodiment 8 may be embodied according to QCLB. Forexample, in the case of QCLBB, whether PRB bundling is applied isdetermined using added proposal 1, and in the case of QCLBA, PRBbundling is applied is determined by RRC signaling.

In addition, RRC signaling of PRB bundling may be transmitted to the UEaccording to PQI states. For example, whether PRB bundling is appliedmay be independently signaled by RRC signaling according to PQI 4 state(state). After the UE acquires PQI information contained in DCI, the UEmaps RRC signaling corresponding to the corresponding PQI state todetermine whether PRB bundling is applied.

FIG. 13 is a diagram illustrating a BS and a UE to which an embodimentof the present invention can be applicable.

When a relay is included in a wireless communication system,communication in backhaul link is performed between the BS and the relayand communication in access link is performed between the relay and theUE. Accordingly, the BS and UE illustrated in FIG. 13 can be replaced bya relay according to a situation.

Referring to FIG. 13, a wireless communication system includes a BS 1310and a UE 1320. The BS 1310 includes a processor 1313, a memory 1314, anda radio frequency (RF) unit 1311 and 1312. The processor 1313 may beconfigured to embody procedures and/or methods proposed according to thepresent invention. The memory 1314 is connected to the processor 1313and stores various information related to an operation of the processor1313. The RF unit 1311 and 1312 is connected to the processor 1313 andtransmits/receives a radio signal. The UE 1320 includes a processor1323, a memory 1324, and an RF unit 1321 and 1322. The processor 1323may be configured to embody procedure and/or methods proposed accordingto the present invention. The memory 1324 is connected to the processor1323 and stores various information related to an operation of theprocessor 1323. The RF unit 1321 and 1322 is connected to the processor1323 and transmits/receives a radio signal. The BS 1310 and/or the UE1320 may have a single antenna or a multiple antenna.

The aforementioned embodiments of the present invention described arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

In the embodiments of the present invention, a specific operationdescribed as being performed by the BS may be performed by an upper nodeof the BS. Namely, it is apparent that, in a network comprised of aplurality of network nodes including a BS, various operations performedfor communication with a UE may be performed by the BS, or network nodesother than the BS. The term ‘BS’ may be replaced with a fixed station, aNode B, an eNode B (eNB), an access point, etc.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, or combinationthereof. In a hardware configuration, the embodiments of the presentinvention may be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention can be implemented by a type of a module, a procedure, or afunction, which performs functions or operations described above.Software code may be stored in a memory unit and then may be executed bya processor.

The memory unit may be located inside or outside the processor totransmit and receive data to and from the processor through variousmeans which are well known.

The detailed description of the preferred embodiments of the presentinvention is given to enable those skilled in the art to realize andimplement the present invention. While the present invention has beendescribed referring to the preferred embodiments of the presentinvention, those skilled in the art will appreciate that manymodifications and changes can be made to the present invention withoutdeparting from the spirit and essential characteristics of the presentinvention. For example, the structures of the above-describedembodiments of the present invention can be used in combination. Theabove embodiments are therefore to be construed in all aspects asillustrative and not restrictive. Therefore, the present inventionintends not to limit the embodiments disclosed herein but to give abroadest range matching the principles and new features disclosedherein.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. Therefore, the present invention intends not tolimit the embodiments disclosed herein but to give a broadest rangematching the principles and new features disclosed herein. It is obviousto those skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

INDUSTRIAL APPLICABILITY

The above-described embodiments of the present invention can be appliedto a wireless communication system such as a user equipment (UE), arelay, a base station (BS), etc.

The invention claimed is:
 1. A method for receiving data from a basestation by a user equipment in a wireless access system, the methodcomprising: receiving information on a plurality of channel stateinformation (CSI) processes for reporting CSI; and receiving the datafrom the base station, wherein the data is processed by using a PhysicalResource Block (PRB) bundling only if all of the plurality of CSIprocesses are configured for reporting a precoding matrix indicator(PMI) and a rank indicator (RI), and wherein the data is processedwithout using the PRB bundling if at least one of the plurality of CSIprocesses is not configured for reporting the PMI and the RI.
 2. Themethod of claim 1, wherein a number of consecutive PRBs included in oneprecoding resource group (PRG) is determined according to a systembandwidth when the data is processed by using the PRB bundling.
 3. Themethod of claim 1, wherein the information on the plurality of CSIprocesses is received using a Radio Resource Control (RRC) signaling. 4.The method of claim 1, further comprising: receiving a demodulationreference signal (DM-RS).
 5. The method of claim 4, further comprising:measuring a channel using the DM-RS.
 6. A method for transmitting datato a user equipment by a base station in a wireless access system, themethod comprising: transmitting information on a plurality of channelstate information (CSI) processes for reporting CSI; and transmittingthe data based on whether or not a Physical Resource Block (PRB)bundling is used, wherein the data is processed by using the PRBbundling only if all of the plurality of CSI processes of the userequipment are configured for reporting a precoding matrix indicator(PMI) and a rank indicator (RI), and wherein the data is processedwithout using the PRB bundling if at least one of the plurality of CSIprocesses of the user equipment is not configured for reporting the PMIand the RI.
 7. The method of claim 6, wherein a number of consecutivePRBs included in one precoding resource group (PRG) is determinedaccording to a system bandwidth if the PRB bundling is used.
 8. Themethod of claim 6, wherein the information on the plurality of CSIprocesses is transmitted using a Radio Resource Control (RRC) signaling.9. The method of claim 6, further comprising: transmitting ademodulation reference signal (DM-RS).
 10. The method of claim 9,wherein the DM-RS is used for the user equipment to measure a channel.11. A user equipment in a wireless access system, the user equipmentcomprising: a radio frequency (RF) unit; and a processor that controlsthe RF unit to: receive information on a plurality of channel stateinformation (CSI) processes for reporting channel state information(CSI) from a base station; and receive data from the base station,wherein the data is processed by using a Physical Resource Block (PRB)bundling only if all of the plurality of CSI processes are configuredfor reporting a precoding matrix indicator (PMI) and a rank indicator(RI), wherein the data is processed without using the PRB bundling if atleast one of the plurality of CSI processes is not configured forreporting the PMI and the RI.
 12. A base station in a wireless accesssystem, the base station comprising: a radio frequency (RF) unit; and aprocessor that controls the RF unit to: transmit, to a user equipment,information on a plurality of channel state information (CSI) processesfor reporting CSI; and transmit, to the user equipment, data based onwhether or not a Physical Resource Block (PRB) bundling is used, whereinthe data is processed by using the PRB bundling only if all of theplurality of CSI processes of the user equipment are configured forreporting a precoding matrix indicator (PMI) and a rank indicator (RI),and wherein the data is processed without using the PRB bundling if atleast one of the plurality of CSI processes of the user equipment is notconfigured for reporting the PMI and the RI.